1. Field of the Invention
This invention relates to a magnetoresistance head for use in magnetic recording devices, VTR""s, etc. The invention relates more particularly to a differential detection type magnetoresistance head.
2. Description of the Related Art
In recent years, the trend of recording tracks toward a decreasing width and that of recording wavelengths toward an increasing frequency have been urging magnetic recording devices such as, for example, hard disc devices to use a further improved recording density. When the width of the recording track is decreased, the sensitivity of the read head is required to be further enhanced because the decrease of the track width entails a decrease in the amount of signal magnetic flux from the magnetic recording medium. As read heads endowed with such high sensitivity, the magnetoresistance heads (hereinafter referred to as xe2x80x9cMR headxe2x80x9d) have been attracting attention.
In the MR heads, the MR heads (GMR heads) of the type using the so-called giant magnetoresistance (hereinafter referred to as xe2x80x9cGMRxe2x80x9d) and having a large rate of change in resistance as compared with the MR heads of the type using the anisotropic magnetoresistance (hereinafter referred to as xe2x80x9cAMRxe2x80x9d) are expected to find recognition as magnetic heads promising high read sensitivity in the future.
For the purpose of improving the linear record resolving power with the shield type MR head which currently prevails in the existing MR heads, the interval or gap between the shield layer possessed of high permeability and the MR element must be decreased. In this case, it is extremely difficult to decrease this interval to or even below 0.1 xcexcm, with maintaining good electric insulation between the shield layer and the MR element. Thus, the shield type MR head has its limit to the improvement of the linear record dissolving power.
As a breakthrough, the so-called dual element type MR head which has two MR elements superposed through the medium of a nonmagnetic intermediate layer has been proposed. This dual element type MR head generates a so-called differential detection type output response by virtue of the phenomenon that it produces a change in the resistance only when the magnetic recording medium applies signal magnetic fields of opposite directions to the two MR elements, whereas it produces no change in the resistance when this medium applies signal magnetic fields of one same direction thereto. The differential detection type MR head has the read resolving power thereof governed by the thickness of the nonmagnetic intermediate layer and, unlike the shield type MR head, requires the two MR elements thereof to be insulated from each other magnetically and not electrically. The differential detection type MR head, therefore, allows the thickness of the nonmagnetic intermediate layer to be notably decreased, for example, to or even below 10 nm. As a result, it is capable of reading an unusually high linear recording density.
Among the conventional differential detection type MR heads is counted the MR head which uses two AMR elements possessing substantially identical magnetoresistance characteristics (MR characteristics). When the AMR elements are used, such operating point bias magnetic fields as rotate their magnetization in opposite directions like about +45xc2x0 and xe2x88x9245xc2x0 from the direction of width of the track are applied. The differential detection type MR head can be realized as a result.
It is suspected, however, that the sensitivity obtainable with the conventional differential detection type MR head using AMR elements will prove insufficient in the near future because the inflow of a signal magnetic field (medium magnetic field) to the upper part of the MR element is not attained when the thickness of the nonmagnetic intermediate layer is decreased. Further, since this MR head necessitates application of the operating point bias magnetic field of the nature mentioned above, it has the problem of encountering difficulty in the impartation and the adjustment of the magnetic field.
The differential detection type MR head using AMR elements is generally constructed by superposing two AMR layers through the medium of a nonmagnetic intermediate layer and forming a pair of electrodes for the supply of a sense current on the upper AMR layer. For the AMR elements in this construction, the angles between the currents and the magnetization constitute themselves an important factor. In order to equalize substantially the MR characteristics of the two AMR elements in the differential detection type MR head, therefore, it is necessary that the characteristics of the two AMR layers themselves be rendered uniform and no angle be formed in the directions of sense currents supplied between the two AMR layers. In the case of the construction in which the electrodes are formed as superposed on one of the AMR layers as described above, the upper and the lower AMR layer produce a difference in the current distribution (as in direction) and the directions of sense currents between these two AMR layers are liable to form an angle. Even when magnetic fields of identical directions are applied to the two AMR layers, therefore, the possibility ensues that the resistance will be varied and this variation will be emitted in the form of a signal. The accidental detection of this erroneous signal possibly results in generation of noise.
Since the conventional differential detection type MR head uses two AMR elements as described above, it entails the problem of readily degrading the sensitivity thereof owing to the fact that the depth of permeation of a signal magnetic field decreases and the fact that the directions of currents between the two AMR layers are inclined toward each other and also the problem of complicating the impartation and the adjustment of the operating point bias magnetic field.
An object of this invention, therefore, is to provide a differential detection type magnetoresistance head which not only possesses improved line resolving power (read resolving power) but also realizes high sensitivity and high S/N ratio under high reliability.
The magnetoresistance head of the present invention is a differential detection type magnetoresistance head which comprises a magnetoresistance element having a laminated structure comprising at least one pair of ferromagnetic layers and an intermediate layer interposed between the pair of ferromagnetic layers and formed of either a nonmagnetic intermediate layer or a granular type ferromagnetic intermediate layer separated into a magnetic region and a nonmagnetic region, the magnetoresistance element part permitting the resistance thereof substantially varied when signal magnetic fields of mutually opposite directions are applied to the pair of ferromagnetic layers, characterized in that the signal magnetic fields are detected by utilizing the variation of resistance based on the giant magnetoresistance due to the spin-dependent scattering in the magnetoresistance element part.
The nonmagnetic intermediate layer as used in the present invention excludes that which is formed of an antiferromagnetic material as well as that which is formed of a ferromagnetic material.
The differential detection type MR head of this invention can be broadly divided into the following three forms. The first form is such that a laminated structure comprising a pair of ferromagnetic layers excelling in the spin-dependent scatting ability (hereinafter referred to as xe2x80x9cGMR ferromagnetic layerxe2x80x9d) and a nonmagnetic intermediate layer interposed between the pair of GMR ferromagnetic layers and possessed of low resistance fit for spin-dependent scattering (hereinafter referred to as xe2x80x9cGMR nonmagnetic intermediate layerxe2x80x9d) is caused to function as a magnetoresistance element part exhibiting a giant magnetoresistance due to the spindependent scattering (hereinafter referred to as xe2x80x9cGMR element partxe2x80x9d).
The second form is such that a laminated structure comprising at least three ferromagnetic layers and nonmagnetic intermediate layers each interposed between the adjacent pairs of the ferromagnetic layers and having the ferromagnetic layers each formed of at least two GMR ferromagnetic layers and at least one low-permeability ferromagnetic layer having the magnetization thereof not substantially varied by a signal magnetic field is caused to function as a GMR element part.
The third form is such that a granular type ferromagnetic intermediate layer separated into a magnetic region and a nonmagnetic region is caused to function as a GMR element part. In this form, the pair of ferromagnetic layers which are opposed to each other across the granular type ferromagnetic intermediate layer are used as ferromagnetic layers for the detection of a signal magnetic field.
The first form of this invention is desired to generate a ferromagnetic coupling force between the pair of GMR ferromagnetic layers and cause the magnetizations of the two GMR ferromagnetic layers to be arranged in substantially identical directions with a signal magnetic field in a state of zero. The ferromagnetic coupling force between the two GMR ferromagnetic layers can be adjusted to a required magnitude by controlling the thickness of the nonmagnetic intermediate layer. The parallel arrangement of the directions of magnetizations may be attained by application of a bias magnetic field. The impartation of the bias magnetic field can be effected by causing a hard ferromagnetic film such as of CoPt or an antiferromagnetic film such as of FeMn or NiO to be disposed closely to or superposed on the GMR element part.
Incidentally, in the standard conventional GMR multilayered film which has ferromagnetic layers and nonmagnetic intermediate layers superposed alternately and utilizes the antiferromagnetic coupling between the ferromagnetic layers, the magnetizations of the ferromagnetic layers are antiferromagnetically arranged with a signal magnetic field in a state of zero. This GMR head cannot be expected to detect a differential detection type signal magnetic field because the magnetizations of the ferromagnetic layers are rotated toward ferromagnetic arrangements and the resistance is consequently varied when signal magnetic fields of equal directions are applied to all the ferromagnetic layers.
The GMR ferromagnetic layers in the differential detection type MR head of the first form may concurrently serve as ferromagnetic layers for the detection of signal magnetic fields. The MR head nevertheless is desired to have a structure such that, apart from the GMR ferromagnetic layers, ferromagnetic layers exclusively used for the detection of signal magnetic fields are severally disposed outside the GMR ferromagnetic layers. Specifically, a laminated structure of GMR ferromagnetic layer/nonmagnetic intermediate layer/GMR ferromagnetic layer is used as the GMR element part and ferromagnetic layers for the detection of signal magnetic fields are severally disposed outside the GMR element part. In this case, the GMR ferromagnetic layers and the ferromagnetic layers for the detection of signal magnetic fields are desired to be exchange coupled.
The ferromagnetic layers for the detection of signal magnetic fields mentioned above are desired to be made of a ferromagnetic material possessing higher permeability than the GMR ferromagnetic layers. Desirably, a pair of ferromagnetic layers for the detection of signal magnetic fields be formed protrudingly on the opposed medium surfaces and the GMR element part be made to recede from the opposed surface of the medium. Owing to this arrangement, the GMR element part having the aforementioned laminated structure can be made to function as a substantial gap. In the case of this arrangement, the thickness of the GMR element part is set with due respect to the fact that the GMR element part functions as a substantial gap. In order to accomplish a high recording density of the order of 4 to 10 Gb/in2, for example, it is desired to give the GMR element part a thickness which is in the approximate range of from 10 to 100 nm.
Then, the ferromagnetic layers for the detection of signal magnetic fields are desired to be made of a material which has higher resistance than the GMR ferromagnetic layers, namely a material having resistivity exceeding 100 xcexcxcexa9cm. Owing to the use of this material, the ferromagnetic layers for the detection of signal magnetic fields are enabled to acquire exalted sensitivity because the material is capable of repressing the partial sense current flow into the layers. Further, it is advantageous that the ferromagnetic layers for the detection of signal magnetic fields be formed of ferromagnetic layers which have a larger thickness than the GMR ferromagnetic layers. In this case, the detection of signal magnetic fields is facilitated because such adverse magnetic effects as the degradation of permeability due to the exchange coupling with the GMR ferromagnetic layers can be diminished.
The differential detection type MR head of the first form mentioned above, unlike the conventional GMR multilayered film, enables the magnetizations of the pair of GMR ferromagnetic layers to assume substantially equal directions with the signal magnetic fields in a state of zero as by exerting a ferromagnetic coupling force between the GMR ferromagnetic layers. In the case of a perpendicular magnetic recording medium, for example, since signal magnetic fields of equal directions are applied to the two GMR ferromagnetic layers when the differential detection type MR head departs from the magnetization transition region of the magnetic recording medium, the angle formed by these magnetizations is not varied and substantially no variation occurs in the resistance. When at least the GMR nonmagnetic intermediate layers directly overlie the magnetization transition region of the magnetic recording medium, signal magnetic fields of mutually opposite directions are applied to the two GMR ferromagnetic layers and the magnetizations corresponding thereto are rotated in different directions. As a result, the angle formed by the magnetizations of the two GMR ferromagnetic layers is varied and the resistance is varied largely. Thus, the detection of recorded information can be attained exclusively with the magnetization transition region of the magnetic recording medium. Particularly, the perpendicular magnetic recording medium of the type which produces an abrupt signal magnetic field transition in the magnetization transition region thereof is enabled to attain read back signals with markedly high resolving power and enjoy generous improvement of linear recording density.
Thus, the differential detection type MR head of the first form, unlike the conventional differential detection type MR head using an AMR element operated by virtue of the resistance which varies proportionately to the angle formed by the current and the magnetization, produces a differential detection by utilizing the resistance variable proportionately to the angle formed by the magnetizations of one pair of GMR ferromagnetic layers instead of relying on the directions of currents. It, therefore, can realize the high read sensitivity which has never been attained by the conventional AMR element. As a result, the noise which originates in the mutual inclination of the directions of currents occurring between the two AMR layers and which has posed a problem to the differential detection type MR head using the conventional AMR element no longer occurs in the differential detection type MR head of the first form. Then, this high read sensitivity can be realized with an extremely simple head structure.
Further, in the differential detection type MR head of the first form, when ferromagnetic layers for the detection of signal magnetic fields are severally disposed outside the GMR element part formed of a laminated structure of GMR ferromagnetic layer/nonmagnetic intermediate layer/GMR ferromagnetic layer, the GMR element part can be made to function as a substantial gap and the gap length, therefore, can be enlarged. As a result, the adaptation of the gap length for the length of the magnetization transition region of the magnetic recording medium and the high read sensitivity can be both fulfilled without contradiction. When the nonmagnetic intermediate layer is made to function by itself to function as a gap, the ratio of variation of the resistance in the GMR element part formed of a laminated structure of GMR ferromagnetic layer/nonmagnetic intermediate layer/GMR ferromagnetic layer will be degraded if the thickness of the nonmagnetic intermediate layer is increased to or even over 5 nm, for example. The result possibly may be that the adaptation of the gap length (of the order of from 10 to 100 nm) for the length of the magnetization transition region of the magnetic recording medium and the high read sensitivity will be simultaneously fulfilled with difficulty.
When the resistance of the ferromagnetic layers for the detection of signal magnetic fields is higher than that of the GMR ferromagnetic layers, the partial sense current flow to the ferromagnetic layers for the detection of signal magnetic fields can be repressed and the sensitivity of the ferromagnetic layers can be enhanced. The direct rotation of magnetizations by the signal magnetic fields of the GMR ferromagnetic layers can be repressed either by forming the ferromagnetic layers for the detection of signal magnetic fields with a ferromagnetic material having higher permeability than the GMR ferromagnetic layers or by causing the GMR element part to recede from the opposed surface of the medium. As a result, the gap length can be more precisely regulated by the thickness of the GMR element part because the rotation of magnetizations of the GMR ferromagnetic layers is predominantly governed as by the exchange coupling bias magnetic fields with the ferromagnetic layers for the detection of signal magnetic fields.
As more specific structures of the GMR element part in the differential detection type MR head of the second form, (1) a structure having a GMR ferromagnetic layer, a GMR nonmagnetic intermediate layer, a low-permeability ferromagnetic layer having the magnetization not substantially varied by a signal magnetic field (hereinafter referred to simply as xe2x80x9clow-permeability ferromagnetic layerxe2x80x9d, a GMR nonmagnetic intermediate layer, and a GMR ferromagnetic layer sequentially superposed in the order mentioned, (2) a structure having a GMR ferromagnetic layer, a GMR nonmagnetic intermediate layer, a low-permeability ferromagnetic layer, a nonmagnetic intermediate layer capable of weakening ferromagnetic coupling between adjacent ferromagnetic layers (hereinafter referred to as xe2x80x9cseparating nonmagnetic intermediate layerxe2x80x9d), a low-permeability ferromagnetic layer, a GMR nonmagnetic intermediate layer, and a GMR ferromagnetic layer sequentially superposed in the order mentioned, and (3) a structure having a low-permeability ferromagnetic layer, a GMR nonmagnetic intermediate layer, a GMR ferromagnetic layer, a separating nonmagnetic intermediate layer, a low-permeability ferromagnetic layer, a GMR nonmagnetic intermediate layer, and a GMR ferromagnetic layer sequentially superposed in the order mentioned may be cited.
In the structure of (1) mentioned above, the magnetization of the low-permeability ferromagnetic layer, for example, is effected in the direction of track width and the magnetizations of the two GMR ferromagnetic layers are inclined by angles of about +45xc2x0 and xe2x88x9245xc2x0, for example, from the direction of track width as by means of a sense current magnetic field with the signal magnetic field in a state of zero. As a result, the detection of signals with high linear response region, low distortion, and high S/N ratio can be carried out. In the structures of (2) and (3), the two low-permeability ferromagnetic layers are desired to be magnetized in directions substantially perpendicular to the medium surface and differing mutually by 180xc2x0. Owing to the magnetization so effected, the magnetization stability of the low-permeability ferromagnetic layers to resist the signal magnetic fields is improved because the low-permeability ferromagnetic layer which is magnetized in the direction opposite to the direction in which the signal magnetic field is applied is exposed to the leak magnetic field emanating from the other low-permeability ferromagnetic layer in the direction tending to cancel the signal magnetic field. These structures are further at an advantage in acquiring a linear magnetic field-resistance characteristics in a wide range of magnetic fields without necessarily causing the magnetizations of the GMR ferromagnetic fields to be inclined in the directions of xc2x145xc2x0.
For the low-permeability ferromagnetic layers mentioned above, a ferromagnetic material having permeability of not more than {fraction (1/10)} of the permeability of the GMR ferromagnetic layer is used. Specifically, hard ferromagnetic materials and semi-hard ferromagnetic materials are concrete examples of the material answering the description. The low-permeability ferromagnetic layers are desired to have permeability of not more than about 100. For the formation of the low-permeability ferromagnetic layers, it is particularly desirable to use a ferromagnetic material which excels in the spin-dependent scattering ability and allows impartation of high coercive force and large uniaxial magnetic anisotropy. The use of this material for the layers is not indispensable when any of the following structures is adopted for the layers.
For the purpose of preventing the magnetizations of the low-permeability ferromagnetic layers from being affected by signal magnetic fields while keeping their large resistance variations intact, such structures as are enumerated below may be adopted, for example. Namely, (a) a structure in which at least the part of the low-permeability ferromagnetic layers is made to recede from the opposed surface of the medium, (b) a structure in which the low-permeability ferromagnetic layers are each formed of an alternate superposition of a ferromagnetic film and a nonmagnetic film and the thicknesses of the nonmagnetic films are optimized and, as a result, a large antiferromagnetic coupling desirably of a coupling magnetic field of not less than about 80000 A/m, a magnitude larger than the signal magnetic field, is enabled by dint of the so-called Ruderman-Kittel-Kasuya-Yoshida (RKKY) exchange interaction to occur between the adjacent ferromagnetic films, (c) a structure in which low-permeability ferromagnetic layers capable of imparting high coercive force and large uniaxial magnetic anisotropy and ferromagnetic films excelling in the spin-dependent scattering ability are superposed, and (d) a structure in which a film adapted for impartation of high coercive force and uniaxial magnetic anisotropy to either of the low-permeability ferromagnetic layers of the construction of (3) is interposed between this low-permeability ferromagnetic layer and a substrate and, at the same time, a film adapted for impartation of high coercive force and uniaxial magnetic anisotropy to the other low-permeability ferromagnetic layer is used for a separating nonmagnetic intermediate layer destined to form the foundation for this other low-permeability ferromagnetic layer may be cited.
In the differential detection type MR head of the second form described above, when the magnetizations of the two GMR ferromagnetic films are rotated in the same direction in conformity to the signal magnetic fields generated in the same direction, the angles formed by the magnetizations relative to the low-permeability ferromagnetic layers opposed to each other across the GMR nonmagnetic intermediate layer increase on the one GMR ferromagnetic layer side and decrease on the other GMR ferromagnetic layer side. As a result, the signal magnetic fields are not detected because the resistance is not varied substantially. When the signal magnetic fields are applied in opposite directions on the two GMR ferromagnetic layers, the angles formed by the magnetizations of the two GMR ferromagnetic layers respectively with the low-permeability ferromagnetic layers opposed to each other across the GMR nonmagnetic intermediate layer are simultaneously increased or decreased. As a result, notably large resistance variations are caused by the spin-dependent scattering. In other words, a differential detection type signal detection system of high sensitivity using GMR elements can be realized. Likewise, in the differential detection type MR head of this second form, the current distribution within the laminated structure avoids exerting an effect on sensitivity or other factors because the differential detection is generated by fundamentally utilizing the resistance which varies with the angles formed by the two magnetizations.
Further, the gap length can be regulated by the interval between the two GMR ferromagnetic layers because the part of GMR nonmagnetic intermediate layer, low-permeability ferromagnetic layer, and separating nonmagnetic intermediate layer which intervenes between the two GMR ferromagnetic layers has notably small permeability with respect to the signal magnetic fields. A proper gap length of the order of from 10 to 100 nm can be fixed proportionately to the linear recording density as by adjusting the thickness of the separating nonmagnetic intermediate layer. All the factors mentioned above boil down to a conclusion that a differential detection type MR head of high sensitivity and high S/N ratio can be realized and the linear recording density can be improved as well.
In the differential detection type MR head of the third form, a granular type ferromagnetic intermediate layer which is divided into a magnetic region formed mainly of Co, Ni, Fe, etc., for example, and a nonmagnetic region formed of Cu, Au, Ag, and alloys thereof, for example, is used. As ferromagnetic layers for the detection of signal magnetic fields which are opposed to each other across this granular type ferromagnetic intermediate layer, those which are used in the structure of the first form are used.
In the differential detection type MR head of the third form, since the granular type ferromagnetic intermediate layer serving as a GMR element part is equivalent to a substantial gap, the thickness of the GMR element part is to be set in consideration of this equivalency. To attain a high recording density falling in the approximate range of from 4 to 10 Gb/in2, for example, the thickness of the granular type ferromagnetic intermediate layer is desired to be selected in the approximate range of from 10 to 100 nm. The granular type ferromagnetic intermediate layer and the ferromagnetic layers for the detection of signal magnetic fields are desired to be exchange coupled across the interfaces thereof. In the same manner as in the structure of the first form, the GMR element part or the granular type ferromagnetic intermediate layer is desired to be withdrawn from the opposed surface of the medium.
The magnetizations within the granular type ferromagnetic intermediate layer are desired to assume substantially equal directions with the signal magnetic fields in a state of zero. The desire is satisfied as by inducing a ferromagnetic coupling force between the pair of ferromagnetic layers for the detection of signal magnetic fields or applying a bias magnetic field thereto. The ferromagnetic coupling force between the pair of ferromagnetic layers for the detection of signal magnetic fields can be adjusted to a necessary magnitude by regulating the state of dispersion of a magnetic region in a nonmagnetic region within the granular type ferromagnetic intermediate layer, specifically by controlling the interval between the adjacent magnetic regions. The bias magnetic field can be imparted as required by causing a hard ferromagnetic film formed of CoPt to be disposed closely to or superposed on or an antiferromagnetic film formed of FeMn or NiO to be disposed closely to the granular type ferromagnetic intermediate layer.
In the differential detection type MR head of the third form, owing to the exchange coupling between the ferromagnetic layers for the detection of signal magnetic fields and the granular type ferromagnetic intermediate layer, the magnetization of the magnetic region of the granular type ferromagnetic intermediate layer gains in susceptibility to the force exerted in the same direction as the magnetization of the ferromagnetic layers for the detection of signal magnetic fields in proportion as the proximity thereof to the ferromagnetic layers for the detection of signal magnetic fields grows. When the magnetizations of the two ferromagnetic layers for the detection of signal magnetic fields fall in substantially equal directions (as when signal magnetic fields are substantially in a state of zero), the magnetizations of the magnetic region of the granular type ferromagnetic intermediate layer assume uniform directions. When signal magnetic fields of equal directions are applied to the two ferromagnetic layers for the detection of signal magnetic fields, therefore, the magnetizations of the magnetic region of the granular type ferromagnetic intermediate layer are rotated as kept in equal directions because the magnetizations of the two ferromagnetic layers for the detection of signal magnetic fields are rotated as kept in equal directions. As a result, the resistance is not varied. When signal magnetic fields of opposite directions are applied instead to the two ferromagnetic layers for the detection of signal magnetic fields, the magnetizations of the magnetic region of the granular type ferromagnetic intermediate layer are rotated in directions differing mutually by 180xc2x0 near the interfaces with the ferromagnetic layers for the detection of signal magnetic fields proportionately to the rotation of the magnetizations of the ferromagnetic layers for the detection of signal magnetic fields. Since the parts of the magnetizations of the magnetic region which deviate from the equal directions increase, the resistance of the granular type ferromagnetic intermediate layer increases. As a result, the differential detection type MR head can be realized because the detection of signals is attained only when signal magnetic fields of opposite directions are applied.
Further, since the granular type ferromagnetic intermediate layer has small permeability, it is allowed to function as a substantial gap. The reading of signal magnetic fields with high sensitivity and high resolution, therefore, is accomplished by adjusting the thickness of the granular type ferromagnetic intermediate layer to a proper magnitude fit for linear recording density. Further by causing the granular type ferromagnetic intermediate layer to recede from the opposed surface of the medium, the signal magnetic fields which are directly sensed by the granular type ferromagnetic intermediate layer (GMR element part) grow weak and the gap length can be regulated more precisely by the thickness of the granular type ferromagnetic intermediate layer. In the differential detection type MR head of the third form likewise, the current distribution within the laminated structure exerts no adverse effect on sensitivity and other factors because the differential detection is principally implemented by utilizing the resistance which varies with the angles to be formed by the magnetizations of the magnetic region of the granular type ferromagnetic intermediate layer.